Department of Genetics, Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, Pretoria, 0002, South Africa.
BMC Plant Biol. 2011 Dec 1;11:173. doi: 10.1186/1471-2229-11-173.
NAC domain transcription factors initiate secondary cell wall biosynthesis in Arabidopsis fibres and vessels by activating numerous transcriptional regulators and biosynthetic genes. NAC family member SND2 is an indirect target of a principal regulator of fibre secondary cell wall formation, SND1. A previous study showed that overexpression of SND2 produced a fibre cell-specific increase in secondary cell wall thickness in Arabidopsis stems, and that the protein was able to transactivate the cellulose synthase8 (CesA8) promoter. However, the full repertoire of genes regulated by SND2 is unknown, and the effect of its overexpression on cell wall chemistry remains unexplored.
We overexpressed SND2 in Arabidopsis and analyzed homozygous lines with regards to stem chemistry, biomass and fibre secondary cell wall thickness. A line showing upregulation of CesA8 was selected for transcriptome-wide gene expression profiling. We found evidence for upregulation of biosynthetic genes associated with cellulose, xylan, mannan and lignin polymerization in this line, in agreement with significant co-expression of these genes with native SND2 transcripts according to public microarray repositories. Only minor alterations in cell wall chemistry were detected. Transcription factor MYB103, in addition to SND1, was upregulated in SND2-overexpressing plants, and we detected upregulation of genes encoding components of a signal transduction machinery recently proposed to initiate secondary cell wall formation. Several homozygous T4 and hemizygous T1 transgenic lines with pronounced SND2 overexpression levels revealed a negative impact on fibre wall deposition, which may be indirectly attributable to excessive overexpression rather than co-suppression. Conversely, overexpression of SND2 in Eucalyptus stems led to increased fibre cross-sectional cell area.
This study supports a function for SND2 in the regulation of cellulose and hemicellulose biosynthetic genes in addition of those involved in lignin polymerization and signalling. SND2 seems to occupy a subordinate but central tier in the secondary cell wall transcriptional network. Our results reveal phenotypic differences in the effect of SND2 overexpression between woody and herbaceous stems and emphasize the importance of expression thresholds in transcription factor studies.
NAC 结构域转录因子通过激活众多转录调控因子和生物合成基因，启动拟南芥纤维和导管的次生细胞壁生物合成。NAC 家族成员 SND2 是纤维次生细胞壁形成主要调控因子 SND1 的间接靶标。先前的研究表明，过量表达 SND2 会导致拟南芥茎中纤维细胞次生细胞壁厚度特异性增加，并且该蛋白能够反式激活纤维素合酶 8（CesA8）启动子。然而，SND2 调控的基因全谱尚不清楚，其过表达对细胞壁化学性质的影响也尚未得到探索。
我们在拟南芥中过表达了 SND2，并对茎化学成分、生物量和纤维次生细胞壁厚度进行了分析。选择CesA8 上调的株系进行全转录组基因表达谱分析。我们发现，该株系中与纤维素、木聚糖、甘露聚糖和木质素聚合相关的生物合成基因上调，与根据公共微阵列数据库中与天然 SND2 转录本的显著共表达一致。仅检测到细胞壁化学性质的微小变化。除 SND1 外，转录因子 MYB103 在 SND2 过表达植物中也上调，并且我们检测到与最近提出的启动次生细胞壁形成的信号转导机制相关的基因上调。几个具有明显 SND2 过表达水平的 T4 纯合子和 T1 半合子转基因株系显示出对纤维壁沉积的负面影响，这可能是由于过度表达而不是共抑制间接引起的。相反，在桉树茎中过表达 SND2 会导致纤维横截面细胞面积增加。
本研究支持 SND2 在调节纤维素和半纤维素生物合成基因方面的功能，除了那些参与木质素聚合和信号转导的基因。SND2 似乎在次生细胞壁转录网络中占据次要但核心的层次。我们的结果揭示了 SND2 过表达在木本和草本茎中的表型差异，并强调了在转录因子研究中表达阈值的重要性。
